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Windload on Glass Railings per IBC 1609.1 applicable and ASCE-7

tbz

tbz

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Good evening all,

I am working with a client's engineer on a project and they are hung up on the following question that has been sent in to ASCE-7, with a response that they will have a subcommittee get back to us, well I am not sure when that will be so I am posting the question below looking for input and possibly a direction to another forum that can help.

The engineer's question:

"We are designing a railing for balconies and other areas on a shoreline building in New Jersey. The railing includes glass infill panels. The 2015 version of the NJ building code section 1609.1 Applications reads as follows:

1609.1 Applications

Buildings, structures and parts thereof shall be designed to withstand the minimum wind loads prescribed herein. Decreases in wind loads shall not be made for the effect of shielding by other structures.

1609.1.1 Determination of Wind Loads
Wind loads on every building or structure shall be determined in accordance with Chapters 26 to 30 of ASCE 7 or provisions of the alternate all-heights method in Section 1609.6. The type of opening protection required, the ultimate design wind speed, Vult, and the exposure category for a site is permitted to be determined in accordance with Section 1609 or ASCE 7. Wind shall be assumed to come from any horizontal direction and wind pressures shall be assumed to act normal to the surface considered.

The 2010 version of ASCE7 (ASCE 7-10 ) is the applicable ASCE 7 standard. Because the glass forms a substantial part of the railing surface, and because wind loads in NJ at the coastline are substantial, we are examining the effects of the wind on the railing.

I have not found any specific way to address a glass railing at a balcony in ASCE7-10. A glass railing at a roof top would certainly have to be addressed as a parapet, applying the positive wall pressure and the negative roof pressure in combination. Because there is a building a few feet behind our glass railing, intuitively, this same approach may be excessive if applied to the glass at the balcony. Furthermore, in our project we have been provided only with the wall (components and cladding) wind loads and a roof wind load. There are no wind loads generated for uplift/suction at the balconies. Each balcony is relatively small I relation to the building (approximately 10 feet wide, projecting 14 inches from the building façade).


Can you provide guidance as to which provisions of ASCE 7 apply to determine the wind load for a glass railing at an apartment balcony?"

That is the question above and some more background information below:
  • IBC 2015 NJ - ASCE 7-10
  • Basic Wind Speed = 121 MPH
  • Risk category - II, Exposure D
Thanks for any help in advance - Tom
 
I'll look when I get to work tomorrow. We have this situation regularly on high and mid-rises along with some SFR's in a basic wind speed of 170mph and exposure D. Some of the products have NOA's but the pressures change based on mean roof height and where the railing will be. We had the engineers look at this 2 ways, the railing anchoring and the thickness of the safety glass. We have a lot of that recently.
 
I doubt you will find any wind provisions addressing glass railings. The wind loads are the same if this railing is made of steel or glass. Treat the railing the same as you would a parapet or fence. On a project where their was a wind consultant the consultant could provide wind pressures.

Do a calculation of what you see as the upper bound. If there are no major cost problems with this design use it and do not spend additional time trying to find the "right" answer "
 
It would be safest to design for the higher of the wind load acting inward or the handrail load in 1607. The building code requires guards to be 42" high. They would be considered Components & Cladding, and it won't take much wind load to exceed the 50 PLF railing load, especially at the corner of a building.

I don't think it's necessary to add the 2 loads, because codes aren't designed to protect somebody stupid enough to be standing on a balcony during a Cat 5 hurricane!
 
Mark K said:
I doubt you will find any wind provisions addressing glass railings. The wind loads are the same if this railing is made of steel or glass. Treat the railing the same as you would a parapet or fence. On a project where their was a wind consultant the consultant could provide wind pressures.

Do a calculation of what you see as the upper bound. If there are no major cost problems with this design use it and do not spend additional time trying to find the "right" answer "
Click to expand...

Mark,

Are you saying that glass railings are not required to meet 1609.1?

Under 1609.1 Applications it says "Buildings, structures and parts thereof", is not the railing a (Part Thereof) of the buildings exterior?

HEre is some more basic information:

Quoted from the Engineer:

"Wind load on a glass railing must take into account the positive wind load (pressure) on one side of the glass and the negative wind load (suction) on the other side of the glass. Wind load pressures are generated by two methods, Load Resistance Factor Design (LRFD) where the windspeed includes a safety factor, and Allowable Stress Design (ASD) where the wind speed does not include a safety factor, and where the safety factor is included in the allowable material stress."-----

-----"looking at the wind load on one side of the glass only, and calculated the wind speed required to achieve an ASD wind load of 211 psf. The 211 psf is an LRFD value, not an ASD value, and has the safety factor included in it. This LRFD value of 211 includes 51 psf positive pressure and -160 psf negative pressure (suction ) on the opposite side of the glass.

The equivalent ASD value is 0.6 x 211 = 127 PSF (still a very high value).

I am looking to see if the -160 psf(LRFD) value can be reduced."

So can the LRFD be reduced or are we going down the wrong rabbit hole?

Also, the difference is design cost is triple, so the effort is worth the time - Regards Tom
 
You will find that you may need to change the panel spans between vertical supports or increase the glass thickness. Your client's choices will affect both.
 
I am not saying that you do not need to comply with the building code. Be careful not to misrepresent what I said. I am simply saying that in my experience it often makes more sense to adopt a conservative interpretation and get on with the job.

I have not done any calculations and do not intend to do any calculations, but I will suggest that the loads you are reporting are far in excess of the loads that I have seen for railings. I think there is something wrong with the loads you are reporting.

With the parapets 14 inches from the building façade I do not see how you can generate any wind loads on the inner face of the parapet.

I have seen a few reports for projects where a wind tunnel study was done and the results were used to define the wind pressures and the pressures were much less than you infer.

In my experience it is not normal practice to consider the railing loads along with full wind load at the same time, but even if you did that the loads you are reporting appear way too high.

Rather than trying to get this forum to serve as a consultant I recommend that you hire another engineer to review the wind loads.
 
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